Zinc oxide-containing transparent conductive electrode

ABSTRACT

A transparent conductive electrode stack containing a work function adjusted zinc oxide is provided. Specifically, the transparent conductive electrode stack includes a layer of zinc oxide and a layer of a work function modifying material. The presence of the work function modifying material in the transparent conductive electrode stack shifts the work function of the layer of zinc oxide to a higher value for better hole injection into the OLED device as compared to a transparent conductive electrode that includes only a layer of zinc oxide and no work function modifying material.

BACKGROUND

The present disclosure relates to a transparent conductive electrode anda method of forming the same. More particularly, the present disclosurerelates to a zinc oxide-containing transparent conductive electrode, anorganic light emitting diode (OLED) device that includes the zincoxide-containing transparent conductive electrode, and methods offorming the zinc oxide-containing transparent conductive electrode andthe OLED device containing the same.

Organic light emitting diode device technology is emerging as a leadingtechnology for displays and lighting. OLED displays posses keyadvantages including vibrant color, high contrast ratios, wide viewingangles and are flexible over conventional liquid crystal displays(LCDs). Moreover, OLED lighting is much more efficient than incandescentbulbs and has similar efficiency as the nitride based light emittingdiodes (LEDs).

A typical OLED comprises a substrate which is usually made of glass or asimilar transparent material. An anode layer is positioned on thesubstrate. The anode layer can be made of a material having a relativelyhigh work function and is substantially transparent for visible light. Atypical material for the anode layer is indium tin oxide (ITO). A layerof electroluminescent material is positioned on the anode layer, servingas the emitting layer of the OLED. Common materials for forming theemitting layer are polymers such as, for example,poly(p-phenylenvinylene) (PPV) and molecules like tris(8-oxychinolinato)aluminum (Alq₃). In the case of molecules, the emitting layer typicallycomprises several layers of the molecules. A cathode layer of materialhaving a lower work function like aluminum (Al), calcium (Ca) ormagnesium (Mg) is positioned on the emitting layer. During operation ofthe OLED, the cathode layer and the anode layer are connected to a powersupply.

The basic principles of electroluminescence and, thus, of the OLED arethe following: The anode layer and the cathode layer inject chargecarriers, i.e., electrons and holes, into the emitting layer. In theemitting layer, the charge carriers are transported and the chargecarriers of opposite charge form so called excitons, i.e., excitedstates. The excitons decay radiatively into the ground state bygenerating light. The generated light is then emitted by the OLEDthrough the anode layer which is made of transparent material like ITO.The color of the generated light depends on the material used for theorganic emitting layer.

SUMMARY

A transparent conductive electrode stack containing a work functionadjusted zinc oxide electrode is provided. Specifically, a transparentconductive electrode stack is provided that includes, in any order, alayer of zinc oxide and a layer of a work function modifying material.The presence of the work function modifying material in the transparentconductive electrode stack of the present disclosure shifts the workfunction of the layer of zinc oxide to a higher value for better holeinjection into the OLED device as compared to a transparent conductiveelectrode that includes only a layer of zinc oxide and no work functionmodifying material.

In one aspect of the present disclosure, a transparent conductiveelectrode stack is provided. In this aspect of the present disclosurethe transparent conductive electrode stack includes a layer of zincoxide, and a layer of a work function modifying material located on anexposed surface of the layer of zinc oxide.

In another aspect of the present disclosure, an organic light emittingdiode (OLED) device is provided. In this aspect of the presentdisclosure, the OLED device includes a substrate; a transparentconductive electrode stack located on an exposed surface of thesubstrate; a layer of electroluminescent material located above thetransparent conductive electrode stack; and a layer of a cathodematerial located on an exposed surface of the layer ofelectroluminescent material. In accordance with the present disclosure,the transparent conductive electrode stack includes a layer of zincoxide, and a layer of a work function modifying material located on anexposed surface of the layer of zinc oxide.

In a further aspect of the present disclosure, a method of forming atransparent conductive electrode stack is provided. The method offorming the transparent conductive electrode stack includes providing alayer of zinc oxide; and forming a layer of a work function modifyingmaterial on an exposed surface of the layer of zinc oxide.

In yet another aspect of the present disclosure, a method of forming anorganic light emitting diode (OLED) device is provided. The method offorming the OLED device includes providing a substrate; forming atransparent conductive electrode stack on a surface of the substrate,wherein the transparent conductive electrode stack comprises a layer ofzinc oxide and layer of a work function modifying material on an exposedsurface of the layer of zinc oxide; forming a layer ofelectroluminescent material on an exposed surface of the transparentconductive stack; and forming a layer of a cathode material on anexposed surface of the layer of electroluminescent material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation (through a cross sectional view)illustrating a substrate that can be employed in one embodiment of thepresent disclosure.

FIG. 2 is a pictorial representation (through a cross sectional view)illustrating the substrate of FIG. 1 after forming a layer of zinc oxideon an exposed surface of the substrate.

FIG. 3 is a pictorial representation (through a cross sectional view)illustrating the structure of FIG. 2 after forming a layer of a workfunction modifying material on an exposed surface of the layer of zincoxide.

FIG. 4A is a pictorial representation (through a cross sectional view)illustrating an embodiment of the present disclosure in which thetransparent conductive electrode stack includes a layer of zinc oxide,and a layer of a metal oxide.

FIG. 4B is a pictorial representation (through a cross sectional view)illustrating an embodiment of the present disclosure in which thetransparent conductive electrode stack includes a layer of zinc oxide,and a layer of a conductive polymer.

FIG. 4C is a pictorial representation (through a cross sectional view)illustrating an embodiment of the present disclosure in which thetransparent conductive electrode stack includes a layer of zinc oxide, alayer of a conductive polymer and a layer of a metal oxide.

FIG. 4D is a pictorial representation (through a cross sectional view)illustrating an embodiment of the present disclosure in which thetransparent conductive electrode stack includes a layer of zinc oxide,and a layer of a metal.

FIG. 4E is a pictorial representation (through a cross sectional view)illustrating an embodiment of the present disclosure in which thetransparent conductive electrode stack includes a layer of zinc oxide,and an array of metal dots.

FIG. 5 is a pictorial representation (through a cross sectional view)illustrating the structure of FIG. 3 after forming a layer of anelectroluminescent material on an exposed surface of the layer of workfunction modifying material.

FIG. 6 is a pictorial representation (through a cross sectional view)illustrating the structure of FIG. 5 after forming a layer of a cathodematerial on an exposed surface of the layer of electroluminescentmaterial.

DETAILED DESCRIPTION

The present disclosure, which provides a zinc oxide-containingtransparent conductive electrode, an organic light emitting diode (OLED)device that includes the zinc oxide-containing transparent conductiveelectrode, and methods of forming the zinc oxide-containing transparentconductive electrode and the OLED device containing the same, will nowbe described in greater detail by referring to the following discussionand drawings that accompany the present application.

It is noted that the drawings of the present application are providedfor illustrative purposes and, as such, they are not drawn to scale. Inthe drawings and the description that follows, like materials arereferred to by like reference numerals. For purposes of the descriptionhereinafter, the terms “upper”, “lower”, “right”, “left”, “vertical”,“horizontal”, “top”, “bottom”, and derivatives thereof shall relate tothe components, layers and/or materials as oriented in the drawingfigures which accompany the present application.

In the following description, numerous specific details are set forth,such as particular structures, components, materials, dimensions,processing steps and techniques, in order to provide a thoroughunderstanding of the present disclosure. However, it will be appreciatedby one of ordinary skill in the art that the present disclosure may bepracticed with viable alternative process options without these specificdetails. In other instances, well-known structures or processing stepshave not been described in detail in order to avoid obscuring thevarious embodiments of the present disclosure.

In current OLED displays and lighting technologies, indium tin oxidetransparent conductive electrodes are used as an anode. Such an OLEDconfiguration has the following disadvantages. Transparent conductiveelectrodes containing indium tin oxide include the rare earth metalindium which is an expensive material thus increasing the cost ofmanufacturing indium tin oxide-containing OLED devices. Indium tinoxide-containing OLED devices easily fail after bending and are thus notsuitable for flexible applications. Also, indium tin oxide is toxic andalternative materials for transparent conductive electrodes are thusneeded.

In the present disclosure, the drawbacks mentioned above with respect toconventional indium tin oxide transparent conductive electrodes havebeen obviated by providing a zinc oxide-containing transparentconductive electrode. Specifically, a transparent conductive electrodestack is provided that includes a layer of zinc oxide and layer of awork function modifying material. The presence of the work functionmodifying material in the transparent conductive electrode stack of thepresent disclosure shifts the work function of the layer of zinc oxideto a higher value for better hole injection into the OLED device ascompared to a transparent conductive electrode that includes only alayer of zinc oxide and no work function modifying material.

Although the following description illustrates the transparentconductive electrode stack of the present disclosure as a component ofan OLED device, the zinc oxide-containing transparent conductiveelectrode of the present disclosure is not limited to being used in onlysuch a device. Instead, the zinc oxide-containing transparent conductiveelectrode of the present disclosure can be used in other types ofdevices such as, for example, photovoltaic devices, solar cells, flatpanel displays or touch screens.

Also, and although the present disclosure illustrates and describes thatthe transparent conductive electrode stack contains a bottom zinc oxidelayer and an upper layer(s) of work function modifying material(s), thepresent disclosure is not limited to only such an embodiment. Instead,and in some embodiments of the present disclosure, particularly when thework function modifying material is comprised of a metal or metal dot,the layer of work function modifying material can be located beneath thelayer of zinc oxide.

Referring to FIG. 1, there is illustrated a substrate 10 that can beemployed in one embodiment of the present disclosure. The substrate 10that can be employed in the present disclosure may be rigid or flexibleand may include, for example, a semiconductor material, glass, aceramic, tape, or a plastic. Typically, the substrate 10 that isemployed in the present disclosure is a transparent substrate. In oneembodiment of the present disclosure, the substrate 10 is transparentand is comprised of glass. In another embodiment of the presentdisclosure, the substrate 10 is transparent and is comprised of aplastic. The substrate 10 that is employed in the present disclosure mayhave a thickness from a few hundred microns to a few millimeters. Inanother embodiment, the substrate 10 that is employed may have athickness from a few tens of microns to a few millimeters. The substrate10 can have other thicknesses that are above and/or below the rangesmentioned above.

Referring to FIG. 2, there is illustrated substrate 10 after forming alayer of zinc oxide 12 on an exposed surface of the substrate 10. Insome embodiments, and as illustrated in the drawings of the presentdisclosure, the layer of zinc oxide 12 serves as a transparent bottomelectrode of an OLED device. In other embodiments, the layer of zincoxide 12 can serve as a top electrode of the OLED device. In yet anotherembodiment, the layer of zinc oxide 12 can serve as an electrode ofother types of devices such as, for example, photovoltaic devices, solarcells, flat panel displays, or touch screen devices.

The layer of zinc oxide 12 can be formed utilizing a deposition processincluding, for example, chemical vapor deposition, evaporation, chemicalsolution deposition, and atomic layer deposition. In one embodiment ofthe present disclosure, the layer of zinc oxide 12 can have a thicknessfrom 100 nm to 2000 nm. The layer of zinc oxide 12 can have otherthicknesses that are above and/or below the range mentioned above.

In one embodiment of the present disclosure, the deposited layer of zincoxide 12 has a work function that is typically within a range from 3.5eV to 4.5 eV. It is noted that the work function values for the asdeposited layer of zinc oxide are substantially lower than the Fermilevel of the electroluminescent material. As such, hole injection intothe electroluminescent material using only a zinc oxide layer as anelectrode material is poor. The present disclosure solves this byforming a work function modifying material in proximity to the layer ofzinc oxide 12.

Referring now to FIG. 3, there is illustrated the structure of FIG. 2after forming a layer of a work function modifying material 14 on anexposed surface of the layer of zinc oxide 12. The combination of thelayer of zinc oxide 12 and the layer of work function modifying material14 provides a transparent conductive electrode stack of the presentdisclosure. The presence of the layer of work function modifyingmaterial 14 within the transparent conductive electrode stack of thepresent disclosure increases the work function of the layer of zincoxide and serves as a p-type dopant for the layer of zinc oxide.

Specifically, the layer of work function modifying material 14 withinthe transparent conductive electrode stack of the present disclosureincreases the work function of the layer of zinc oxide 12 such that thework function of the layer of zinc oxide 12 ‘substantially’ matches theFermi level of the layer of electroluminescent material to besubsequently formed atop the transparent conductive electrode stack(i.e., elements 12 and 14). By “substantially matches” it is meant thelayer of zinc oxide 12 when used in conjunction with the layer of workfunction modifying material 14 has a work function that is within lessthan 0.7 eV from the Fermi level of the layer of electroluminescentmaterial. As such, better hole injection into the layer ofelectroluminescent material is provided by employing the transparentconductive electrode stack of the present disclosure.

Also, the layer of work function modifying material 14 within thetransparent conductive electrode stack of the present disclosureincreases the conductivity of the layer of zinc oxide 10 without losingtransmittance.

In one embodiment of the present disclosure, the layer of work functionmodifying material 14 that can be employed in the present disclosure maybe a single layered structure. In another embodiment of the presentdisclosure, the layer of work function modifying material 14 that can beemployed in the present disclosure may be a multilayered structurecomprises at least two distinct layers of work function modifyingmaterials.

The work function modifying material 14 that can be employed in thepresent disclosure can be a metal oxide, a conductive polymer, a metal,an array of metal dots, or any combination thereof including, forexample, a multilayered structure including a metal oxide and aconductive polymer.

Some specific examples of various transparent conductive electrodestacks of the present disclosure are illustrated in FIGS. 4A-4E. Whilethese specific embodiments are shown and described, the transparentconductive electrode stack of the present disclosure is not limited tothose shown in FIGS. 4A-4E.

Specifically, FIG. 4A illustrates an embodiment of the presentdisclosure in which the transparent conductive electrode stack includesa layer of zinc oxide 12 and a layer of a metal oxide 16 as the layer ofwork function modifying material 14. FIG. 4B illustrates an embodimentof the present disclosure in which the transparent conductive electrodestack includes a layer of zinc oxide 12, and a layer of a conductivepolymer 18. FIG. 4C illustrates an embodiment of the present disclosurein which the transparent conductive electrode stack includes a layer ofzinc oxide 12, a layer of a conductive polymer 18 and a layer of a metaloxide 16. FIG. 4D illustrates an embodiment of the present disclosure inwhich the transparent conductive electrode stack includes a layer ofzinc oxide 12, and a layer of a metal 20, while FIG. 4E illustrates anembodiment of the present disclosure in which the transparent conductiveelectrode stack includes a layer of zinc oxide 12, and an array of metaldots 22. In the illustrated embodiments shown in FIGS. 4A-4E, elements16, 18, 20 and 22 serves as the work function modifying material of thepresent disclosure.

When a metal oxide is employed as the work function modifying material14, the metal oxide includes an element from Groups IIIB, VIB, VB, VIIB,VIIB, VIII or IIIA of the Periodic Table of Elements, with the provisothat the metal oxide is other than zinc oxide. Illustrated examples ofmetal oxides that can be employed in the present disclosure as workfunction modifying material 14 include, but are not limited to, MoO₃,W₂O₅, Al₂O₃, or combinations and multilayers thereof. The metal oxidecan be formed utilizing any well known deposition process including, forexample, evaporation, chemical solution deposition, chemical vapordeposition, and sputtering.

When a conductive polymer is employed as the layer of work functionmodifying material 14, the conductive polymer (which can be referred toas an intrinsically conductive polymer) includes an organic polymer thatconducts electricity. Examples of conductive polymers that can beemployed in the present disclosure include, for example, aromaticcompounds containing no heteroatoms, aromatic compounds containing anitrogen heteroatom, aromatic compounds containing a sulfur heteroatom,polymeric compounds containing double bonds and/or aromatic compoundsthat also contain double bonds. In some embodiments of the presentdisclosure, the conductive polymers that can be employed in the presentdisclosure as the layer of work function modifying material 14 areselected from polyanilines andpoly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) or PEDOT:PSS forshort.

The conductive polymer can be formed utilizing any well known depositionprocess including, for example, evaporation, chemical solutiondeposition, spin-coating, or dip coating. In some embodiments of thepresent disclosure the application of a conductive polymer directly to asurface of a layer of zinc oxide 12 can also form a uniform contact withthe layer of zinc oxide 12.

When a metal or metal dot is employed as the layer of work functionmodifying material 14, the metal or metal dot includes an element fromGroups IIIB, IVB, VB, VIIB, VIIB, VIII or IIIA of the Periodic Table ofElements. Illustrated examples of metals or an array of metal dotsinclude, but are not limited to, Au, Pd, Pt, W, Ag, and/or Al.

A metal film can be formed utilizing any well known deposition processincluding, for example, evaporation, chemical solution deposition,chemical vapor deposition, and sputtering. An array of metal dots can beformed by first depositing a metal and then patterning the same byphotolithography and etching. Alternatively, an array of metal dots canbe formed by deposition of a metal precursor through a mask whichcontains a preconfigured pattern formed therein.

In one embodiment of the present disclosure, the layer of work functionmodifying material 14 can have a thickness from 1 nm to 80 nm. Inanother embodiment, the layer work function modifying material 14 canhave a thickness from 5 nm to 50 nm. The layer of work functionmodifying material 14 can have other thicknesses that are above and/orbelow the ranges mentioned above.

Referring to FIG. 5, there is illustrated the structure of FIG. 3 afterforming a layer of an electroluminescent material 24 above thetransparent conductive electrode stack containing elements 12 and 14.The layer of electroluminescent material 24 that is employed in thepresent disclosure includes any organic material or multilayered stackof organic materials including, for example, organometallic chelates,conductive polymers, fluorescent dyes, phosphorescent dyes andconjugated dendrimers, that emits light in response to an electriccurrent. Examples of organic materials that can be used as theelectroluminescent material 24 include, but are not limited to,poly(p-phenylenvinylene) (PPV), poly(naphthalene vinylenes) (PNVs),tris(2-phenyl pyridine)iridium (Ir(ppy)₃), and tris(8-oxychinolinato)aluminum (Alq₃).

The layer of electroluminescent material 24 can be formed byconventional techniques including, for example, spin-on coating, dipcoating, immersion, and chemical vapor deposition. Typically, and in oneembodiment, the thickness of the layer of electroluminescent material 24ranges from a few nm to a few hundred nm. Other thicknesses, includingthose above and/or below the aforementioned range can also be employed.

Referring now to FIG. 6, there is illustrated the structure of FIG. 5,after formation of a layer of a cathode material 26 on an exposedsurface of the layer of electroluminescent material 24. The layer ofcathode material 26 can serve as an upper electrode of the OLED of thepresent disclosure. The layer of cathode material 26 that is employed inthe present disclosure includes a material or a multilayered stack ofmaterials having a lower work function than the transparent conductiveelectrode stack of doped graphene 12. In one embodiment of the presentdisclosure, the layer of cathode material 18 can be comprised ofaluminum (Al), calcium (Ca) and/or magnesium (Mg). In some embodiments,the layer of cathode material may comprise a stack of LiF and Al.

The layer of cathode material 26 can be formed utilizing any depositionprocess including for example, thermal evaporation and sputtering. Insome embodiments, the deposition process is performed through a shadowmask. Typically, and in one embodiment, the thickness of the layer ofcathode material 26 ranges from 20 nm to 100 nm. Other thicknesses,including those above and/or below the aforementioned range can also beemployed.

The transparent conductive electrode stack of the present disclosurewhich comprises a combination of the layer of zinc oxide 12 and thelayer of work function modifying material 14 is typically less toxicthan a conventional ITO transparent conductive electrode. Also,transparent conductive electrode stacks comprised of the combination ofthe layer of zinc oxide 12 and the layer of work function modifyingmaterial 14 are less expensive to fabricate than are their ITOcounterparts. Further, transparent conductive electrode stacks comprisedof the combination of the layer of zinc oxide 12 and the layer of workfunction modifying material 14 are more compatible with plasticsubstrates and thus can be used in a wide variety of display andlighting applications. Furthermore, transparent conductive electrodestacks comprised of the combination of the layer of zinc oxide 12 andthe layer of work function modifying material 14 typically have a highermechanical strength than their ITO counterpart electrodes. Moreover, thetransparent conductive electrode stacks comprised of combination of thelayer of zinc oxide 12 and the layer of work function modifying material14 are chemically stable. By “chemically stable” it is meant that thetransparent conductive electrode stack of the present disclosure canendure processing steps which include a strong acid, base, and/orsolvent, and maintain its structural integrity.

When used as a component of an OLED device, the transparent conductiveelectrode stacks comprised of the combination of the layer of zinc oxide12 and the layer of work function modifying material 14 can provide anOLED device that has electrical properties that are similar to, and insome embodiments, slightly improved as compared with, an OLED devicecontaining a conventional ITO electrode.

In some embodiments, the transparent conductive electrode stackscomprised of the combination of the layer of zinc oxide 12 and the layerof work function modifying material 14 can provide an OLED device thathas a same, or slightly higher quantum efficiency as compared to an OLEDdevice containing a conventional ITO electrode. In some cases, thetransparent conductive electrode stacks comprised of the combination ofthe layer of zinc oxide 12 and the layer of work function modifyingmaterial 14 can provide an OLED device that has a same quantumefficiency as compared to an OLED device containing a conventional ITOelectrode. The external quantum efficiency without any out-couplingscheme is more than 20%.

While the present disclosure has been particularly shown and describedwith respect to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formsand details may be made without departing from the spirit and scope ofthe present disclosure. It is therefore intended that the presentdisclosure not be limited to the exact forms and details described andillustrated, but fall within the scope of the appended claims.

What is claimed is:
 1. A transparent conductive electrode stack comprising: a layer of zinc oxide; and a layer of a work function modifying material located on an exposed surface of the layer of zinc oxide, wherein said layer of work function modifying material comprises at least one of a metal oxide containing an element selected from the group consisting of Group IIIB and VIIB of the Periodic Table of Elements, a metal containing an element selected from IIIB, IVB, VB, VIB, VIIB, VIII and IIIA of the Periodic Table of Elements, and an array of metal dots containing an element selected from IIIB, IVB, VB, VIB, VIIB, VIII and IIIA of the Periodic Table of Elements.
 2. The transparent conductive electrode of claim 1, wherein said layer of work function modifying material is located above said layer of zinc oxide.
 3. The transparent conductive electrode of claim 1, wherein said layer of work function modifying material is located beneath said layer of zinc oxide.
 4. The transparent conductive electrode of claim 1, wherein said layer of zinc oxide is located on a transparent substrate.
 5. An organic light emitting diode (OLED) device comprising: a substrate; a transparent conductive electrode stack located on an exposed surface of the substrate; a layer of electroluminescent material located above the transparent conductive electrode stack; a layer of a cathode material located on an exposed surface of the layer of electroluminescent material, wherein said transparent conductive electrode stack comprises a layer of zinc oxide, and a layer of a work function modifying material located on an exposed surface of the layer of zinc oxide, wherein said layer of work function modifying material comprises at least one of a metal oxide containing an element selected from the group consisting of Group IIIB and VIIB of the Periodic Table of Elements, a metal containing an element selected from IIIB, IVB, VB, VIB, VIIB, VIII and IIIA of the Periodic Table of Elements, and an array of metal dots containing an element selected from IIIB, IVB, VB, VIB, VIIB, VIII and IIIA of the Periodic Table of Elements.
 6. The OLED device of claim 5, wherein said layer of a work function modifying material is located above said layer of zinc oxide.
 7. The OLED device of claim 5, wherein said layer of electroluminescent material is selected from the group consisting of poly(p-phenylenvinylene) (PPV), poly(naphthalene vinylenes) (PNVs), tris(2-phenyl pyridine)iridium (Ir(ppy)₃), and tris (8-oxychinolinato) aluminum (Alq₃).
 8. The OLED device of claim 5, wherein said layer of cathode material comprises aluminum (Al), calcium (Ca), magnesium (Mg) or a combination thereof.
 9. The OLED device of claim 5, wherein said substrate is transparent.
 10. The OLED device of claim 9, wherein said substrate is comprised of glass or a plastic.
 11. A transparent conductive electrode stack comprising: a layer of zinc oxide; and a layer of a work function modifying material located on an exposed surface of the layer of zinc oxide, wherein said layer of work function material comprises a material stack of a metal oxide and a conductive polymer.
 12. The transparent conductive electrode stack of claim 11, wherein said metal oxide contains an element selected from the group consisting of Group IIIB, IVB, VB, VIIB, and IIIA of the Periodic Table of Elements.
 13. The transparent conductive electrode stack of claim 11, wherein said metal oxide consists of MoO₃.
 14. The transparent conductive electrode stack of claim 11, wherein said metal oxide comprises W₂O₅ or A1₂O₃.
 15. The transparent conductive electrode stack of claim 11, wherein said conductive polymer is selected from aromatic compounds containing no heteroatoms, aromatic compounds containing a nitrogen heteroatom, aromatic compounds containing a sulfur heteroatom, polymeric compounds containing double bonds and/or aromatic compounds that also contain double bonds.
 16. The transparent conductive electrode of claim 15, wherein said conductive polymer is a polyaniline.
 17. The transparent conductive electrode of claim 15, wherein said conductive polymer is poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate). 